The present invention relates to a control system for a phase controlled rectifier circuit and more particularly to a control system for assuring that electric valves in a phase control rectifier circuit are provided with firing signals at a time when the valves are forward biased.
There are many known power conversion circuits for changing the form of electric power from alternating current to direct current. Such circuits are properly referred to as rectifiers. In some of these circuits the conversion is accomplished by appropriately controlling periodically conducting electric valves that are interconnected between a-c and d-c terminals, the a-c terminals being connected to a system of alternating voltage with which the valve firings are synchronized. A capacitor connected across the d-c terminals serves to filter and smooth the d-c output of the rectifier circuit. In modern practice each valve typically comprises one or more solid state gate controlled switching components known as semiconductor controlled rectifiers or thyristors.
In operation, such a valve has a non-conducting or blocking state, in which it presents very high impedance to the flow of current, and a conducting or turned-on state in which it freely conducts forward current with only a relatively slight voltage drop. It can be switched abruptly from the former state to the latter by the concurrence of a forward bias on its main electrodes (anode at a positive potential with respect to cathode) and a control or trigger signal on its gate. The time at which the valve is turned on, measured in electrical degrees from a cyclically recurring instant at which its anode voltage first becomes positive with respect to its cathode, is known as the "firing angle." The magnitude of the output voltage of the rectifier circuit can be varied by retarding or advancing the firing angle as desired.
Once turned on, a valve will continue conducting until "forward current" is subsequently reduced below a given holding level by the action of the external circuit in which the valve is connected. In a single rectifier circuit, the valve may be turned off by the action of the a-c input voltage becoming less than the d-c output voltage so that the valve is reversed biased and the current through the valve drops to below the necessary holding value. If, however, the filter includes a series inductor connected between the valve and the capacitor, the valve will not turn off until the inductor current goes to zero. Alternately, turn off may occur when the a-c line voltage reverses and transfers current from the valve to a free-wheeling path. The free-wheeling path may comprise a unidirectional conducting device connected across the rectifier's d-c terminals or may comprise a portion of the rectifier circuit itself.
In some applications the load connected to the d-c terminals of the rectifier circuit is variable. If the load is relatively light, whereby the rectifier current is discontinuous, the voltage on the d-c filtering capacitor will tend to approach the peak value of the a-c input voltage. Consequently, a trigger signal applied to the valve may occur at a time at which the valve is reversed biased (cathode at a positive potential with respect to anode) and the valve will not be triggered into conduction. Under light load conditions, the required rectifier current is small and the firing angle to maintain the desired d-c voltage tends to be retarded, i.e., the conduction phase angle of the valves is small and the current through the series inductor to the filter capacitor is discontinuous. Without current flow through the inductor, its voltage drop is essentially zero and the voltage on the capacitor is reflected to the phase controlled rectifier circuit. When the load is increased thereby causing the d-c voltage to begin dropping, the system will attempt to correct the voltage reduction by advancing the firing angle. However, an advanced firing angle can result in a firing pulse occurring at a time at which the valves in the phase controlled rectifier circuit are reverse biased. Accordingly, the d-c voltage cannot be built-up from a light load condition.
One prior art method of overcoming the problem associated with failure of a valve to trigger is to provide a continuous trigger signal from the desired firing time until the next zero crossing of the a-c input voltage. The continuous trigger signal may be either a d-c voltage or a relatively high frequency series of pulses. Both types of trigger signals are in common use. However, continuous triggering of the valve presents several problems in addition to increased cost. For example, the triggering circuit must be isolated from the power circuit using pulse transformers. For continuous triggering, the pulse transformer must be capable of dissipating approximately 100 times more watt-seconds of energy than is required for a single pulse. The power supply required for supplying the continuous trigger signal is also large. Furthermore, solid state valves tend to have increased reverse leakage current when triggered during reverse bias conditions and in addition to overheating are susceptible to thermal runaway.
Accordingly, it is an object of the present invention to provide a control system including a phase controlled rectifier circuit with assured firing without continuous triggering.